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Accretion disc winds in tidal disruption events: ultraviolet spectral lines as orientation indicators

Accretion disc winds in tidal disruption events: ultraviolet spectral lines as orientation indicators
Accretion disc winds in tidal disruption events: ultraviolet spectral lines as orientation indicators
Some tidal disruption events (TDEs) exhibit blueshifted broad absorption lines (BALs) in their rest-frame ultraviolet (UV) spectra, while others display broad emission lines (BELs). Similar phenomenology is observed in quasars and accreting white dwarfs, where it can be interpreted as an orientation effect associated with line formation in an accretion disc wind.We propose and explore a similar unification scheme for TDEs. We present synthetic UV spectra for disc and wind-hosting TDEs, produced by a state-of-the-art Monte Carlo ionization and radiative transfer code. Our models cover a wide range of disc wind geometries and kinematics. Such winds naturally reproduce both BALs and BELs. In general, sight lines looking into the wind cone preferentially produce BALs, while other orientations preferentially produce BELs. We also study the effect of wind clumping and CNO-processed abundances on the observed spectra. Clumpy winds tend to produce stronger UV emission and absorption lines, because clumping increases both the emission measure and the abundances of the relevant ionic species, the latter by reducing the ionization state of the outflow. The main effect of adopting CNO-processed abundances is a weakening of C~{\sc iv}~1550~\AA~ and an enhancement of N \textsc{v}~1240~\AA~ in the spectra. We conclude that line formation in an accretion disc wind is a promising mechanism for explaining the diverse UV spectra of TDEs. If this is correct, the relative number of BAL and BEL TDEs can be used to estimate the covering factor of the outflow. The models in this work are publicly available online and upon request.
accretion, accretion discs, black hole physics, galaxies: nuclei
0035-8711
4914-4929
Parkinson, Edward J.
c1b87057-e577-499a-a3e6-7b31b075d3dc
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Long, Knox S.
2195d0ac-518d-4738-8e89-3e8e7a035a6c
Matthews, James H.
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Higginbottom, Nick
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Sim, Stuart A.
67bb8102-b981-4e2e-9617-8c7806ef1329
Hewitt, Henrietta A.
c4ab711c-bb1c-449f-82c4-51eb625bfd44
Parkinson, Edward J.
c1b87057-e577-499a-a3e6-7b31b075d3dc
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Long, Knox S.
2195d0ac-518d-4738-8e89-3e8e7a035a6c
Matthews, James H.
8aa37525-32b9-460c-bb83-01c89269ac31
Higginbottom, Nick
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Sim, Stuart A.
67bb8102-b981-4e2e-9617-8c7806ef1329
Hewitt, Henrietta A.
c4ab711c-bb1c-449f-82c4-51eb625bfd44

Parkinson, Edward J., Knigge, Christian, Long, Knox S., Matthews, James H., Higginbottom, Nick, Sim, Stuart A. and Hewitt, Henrietta A. (2020) Accretion disc winds in tidal disruption events: ultraviolet spectral lines as orientation indicators. Monthly Notices of the Royal Astronomical Society, 494 (4), 4914-4929. (doi:10.1093/mnras/staa1060).

Record type: Article

Abstract

Some tidal disruption events (TDEs) exhibit blueshifted broad absorption lines (BALs) in their rest-frame ultraviolet (UV) spectra, while others display broad emission lines (BELs). Similar phenomenology is observed in quasars and accreting white dwarfs, where it can be interpreted as an orientation effect associated with line formation in an accretion disc wind.We propose and explore a similar unification scheme for TDEs. We present synthetic UV spectra for disc and wind-hosting TDEs, produced by a state-of-the-art Monte Carlo ionization and radiative transfer code. Our models cover a wide range of disc wind geometries and kinematics. Such winds naturally reproduce both BALs and BELs. In general, sight lines looking into the wind cone preferentially produce BALs, while other orientations preferentially produce BELs. We also study the effect of wind clumping and CNO-processed abundances on the observed spectra. Clumpy winds tend to produce stronger UV emission and absorption lines, because clumping increases both the emission measure and the abundances of the relevant ionic species, the latter by reducing the ionization state of the outflow. The main effect of adopting CNO-processed abundances is a weakening of C~{\sc iv}~1550~\AA~ and an enhancement of N \textsc{v}~1240~\AA~ in the spectra. We conclude that line formation in an accretion disc wind is a promising mechanism for explaining the diverse UV spectra of TDEs. If this is correct, the relative number of BAL and BEL TDEs can be used to estimate the covering factor of the outflow. The models in this work are publicly available online and upon request.

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2004.07727v1 - Author's Original
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Accretion Disc Winds in Tidal Disruption Events - Accepted Manuscript
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More information

Submitted date: 31 January 2020
e-pub ahead of print date: 16 April 2020
Published date: June 2020
Additional Information: 16 pages, 9 figures
Keywords: accretion, accretion discs, black hole physics, galaxies: nuclei

Identifiers

Local EPrints ID: 437788
URI: http://eprints.soton.ac.uk/id/eprint/437788
ISSN: 0035-8711
PURE UUID: f374191a-1793-4815-bb6d-152a9cba064b

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Date deposited: 17 Feb 2020 17:31
Last modified: 21 Jan 2021 17:32

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Contributors

Author: Edward J. Parkinson
Author: Knox S. Long
Author: James H. Matthews
Author: Nick Higginbottom
Author: Stuart A. Sim
Author: Henrietta A. Hewitt

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